Compounds having an absorption in the visible light region are used as optical elements in optical filters for image display devices such as liquid crystal display devices (LCDs), plasma display panels (PDPs), electroluminescence displays (ELDs), cathode ray tube display devices (CRTs), fluorescence display tubes and electric field radiation type displays.
On the other hand, materials in which electrons that have excited by absorbing energy radiate electromagnetic wave as excess energy when the electrons return to a ground state have a wavelength conversion capability due to the difference in energies for absorption and release, and thus have been conventionally used as color conversion dyes (wavelength conversion dyes) in colorants, pigments, optical filters, films for agriculture and the like, and studies have been actively made on organic compounds since wavelengths for absorption and release are controlled easier than in inorganic compounds. Specifically, compounds that radiate absorbed energy as fluorescence are called as fluorescent dyes, and those radiate fluorescence light of visible light are highly practical and can be used for applications such as display devices such as displays, illumination devices such as fluorescent lamps, and markers in biology and medicine.
In general, fine illumination with excellent color rendition (white color that is seen more naturally) is required for illumination devices (white illuminations for home use and the like), and it is considered that it is preferable to mix three primary colors of RGB so as to obtain excellent color rendition, and it is a problem to obtain luminescent with strong RGB. Specifically, an LED illumination generally has a combination of a blue LED and a yellow phosphor, and an approach to replace the yellow phosphor with red and green phosphors so as to improve color rendition was made, but the approach had a problem that it was expensive and poor in energy efficiency. Furthermore, a consideration to absorb only yellow color from fluorescence so as to improve color rendition was also made in the case when red and green phosphors are used, but the approach could not be considered to be sufficient since the brightness of an illumination decreased by only absorbing yellow color.
Patent Literatures 1 to 6 show squarylium compounds and optical filters using the compounds. Furthermore, Patent Literature 7 shows a photoelectric conversion device (solar cell module) using a color conversion material.